This invention relates generally to semiconductor and superconductor technologies, and more particularly the invention relates to a silicon substrate such as an integrated circuit having a superconducting layer thereon. The invention further relates to a method of epitaxially forming on silicon a superconducting layer having good technological properties for electronic devices and circuit applications.
Much research is being pursued in the field of superconductivity in respect to the new classes of ceramic materials such as copper oxides combined with lanthanum, yttrium, barium, thallium, lead, bismuth, calcium, praesedinium, and strontium which have a relatively high superconductivity transition temperature (T.sub.c). The yttrium, barium, and copper oxide materials (Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7) are referred to as 1-2-3 compounds and herein as YBCO.
Recently, work at Stanford University has been pursued to optimize the critical current density of superconducting materials. Materials have been yttrium barium and copper oxide perovskite compounds. The material has good superconducting properties at 77.degree. K. and as a result can be used in combination with liquid nitrogen cooling. Thin film construction of the material is possible through vacuum deposition, either by electron-beam deposition or magnetron sputtering. Many of these planar films still show a limitation in critical current density due to random or non-preferred crystal orientation and grain boundary effects and atmospheric poisoning effects. When the films are deposited on single-crystal strontium titanate with a crystal orientation which is selected to be optimum, the deposited films orient themselves and grow epitaxially on the strontium titanate substrate after heat treatment. These films have exhibited critical current densities of 1.0 to 1.5.times.10.sup.6 amperes per square centimeter at 4.2K which is exceptionally good.
The formation of high temperature superconductor material on silicon substrates has been a recognized goal of researchers for years. Silicon integrated circuits are operable at superconducting temperatures (at present as high as 125.degree. K.). A limitation in realizing ultra high speed semiconductor ICs lies in the interconnection of various circuits in the semiconductor substrate. Conventionally this is accomplished by metal interconnect layers on the surface of the semiconductor substrate. By reducing the resistance of the interconnect metallization, circuit speed of operation can be enhanced.
Superconducting material such as YBCO cannot be deposited directly on silicon because the chemical reaction of silicon with YBCO either completely destroys the superconductivity of the material or significantly depresses the superconducting transition temperature. Numerous attempts have been made in using barrier layers between the silicon and the superconducting material to prevent chemical reactions yet allow the growth of a highly oriented high temperature semiconductor film with a high superconducting temperature. Barrier layers heretofore used include oxides of silicon, zirconium, magnesium, strontium and titanium, magnesium and aluminum, aluminum, lanthanum and aluminum, and others. Yttria stabilized zirconia (YSZ) has been proposed as a barrier layer. However, the superconducting material using these barriers has not had the technological properties necessary for practical utilization. Two of the problems encountered in using these barrier materials lie in contamination from the silicon substrate and oxidation of the silicon substrate during processing.
The present invention provides a silicon substrate having a high temperature superconductor layer epitaxially formed thereon which has the properties necessary for practical applications.